I used to go until my cams changed orientation from where I have them marked, or the feel of the bow changed. But a few years ago I had a yoke loop break right before full draw. Each side of the yoke has half the number of strands, so should be replaced twice as often as bows without yokes I imagine. I estimated I had about 30k shots on them, I obviously pushed them too far. But for my other bows it's pretty obvious when something changes and it's time.
Most of the time 3-4 years depending on how it's holding up. I've been shooting a lot more now since the lock down so I may have to go to 2-3 years. My local shop recommends GAS bowstrings and says they hold up much better than some others, so we'll see. I've had new one on for about 5 months and it still looks great so far after average of 50 shots per day.